Circuit arrangement for regulating transmission measure in sound transmission equipment



July 6, 1937. H. FRIEDRICH 2,035,905

CIRCUIT ARRANGEMENT FOR REGULATING TRANSMISSION MEASURE IN SOUND TRANSMISSION EQUIPMENT Filed Sept. 12, 1.954

INVENTOR HAM J F /0/?/CH ATTORNEY Patented July 6, 1937 PATENT OFFICE CIRCUIT ARRANGEMENT FOR REGULAT- ING TRANSMISSION MEASURE IN SOUND TRANSMISSION EQUIPMENT Hans Friedrich, Berlin, Germany, assignor to Gesellschal't fiir Drahtlose Telegraphic m. b. 11., Berlin, Germany, a corporation of Germany Application September 12, 1934, Serial No. 743,672 In Germany May 11, 1933 2 Claims.

In a great number of branches in the arts of electro-acoustics, it is desirable to have available circuit means adapted to regulate the transmission gain. Regulator means of this kind serve for the compensation of .tading, amplitude limitation and adjustment of dynamics (1. e., ratio of. maximum and minimum volume). Wherever by over-modulations of some parts of the transmission path disturbances are liable to happen, for instance, inthe voice-modulation of transmitters, recording of phonograph, records and reproduction thereof, in sound-film recording and reproduction thereof as well as in the reproduction by broadcast receiver apparatus, it

seems that regulator devices of the said sort are suitable.

Circuit arrangements of thekind known in the earlier art designed for amplification control invariably have recourse to variation of slope of to mutual conductance of grids equipped with one or with more grids for gain control. Hence, this requires at all events a greater number of tubes in the amplifier. The increase in cost of the entire equipment thereby occasioned has heretofore been an important factor tending to prevent the wider introduction in practice of regulating means of the said sort. a

Now, it has been discovered that a regulator scheme that measures up to all practical requirements, while yet being very simple in construction and low in cost may be obtained by that the comparatively expensive and delicate amplifier tubes are replaced by simpler non-linear resistances such as dry (electrolytic) rectifiers, detec- 5 tors, hot-cathode rectifiers, choke-coils comprising iron-cores, etc. These non-linear resistances are provided with biasing voltage which governs the working point that is desired in any given instance in the characteristic connecting the current and voltage.

Figure 1 shows a rectifier characteristic for gain control.

Figure 2 shows a circuit arrangement embodying the use of non-linear resistances for gain control.

Figure 3 shows a modification of Figure 2.

Figure 4 shows a further modification of Fi ure 2.

60 Fig. 1 shows a rectifier characteristic of the said sort useful for gain control according to this invention. In order to prevent non-linear distortions from arising in such a regulation scheme the altematlng voltage at the given working point must be chosen so as to be adapted to the respective biasing potential of the non-linear resistance.

Fig. 2 shows the fundamental scheme of a circuit arrangement embodying the use of resistances of non-linear action for gain control. From the source of alternating voltage T, such as a microphone, pick-up or the like, the internal resistance of'which shall be denoted by Ri, the ensuing voltage is impressed by way of condenser C upon the oppositely connected rectifiers GI and G2. The variable potential, e. g., of battery B serves for determining the working point for the rectifiers and thus for volume control. In the presence of a high biasing potential the working point must be shifted into a region of greater slope and thus higher gain than when the biasing voltage is lower. Across the terminals of resistance Ra is obtained the regulated alternating voltage and the same is then fed, optionally by way of amplifiers, to the loudspeaker, the sound recorder device, the transmitter or other sound transmission means. What is presupposed for the satisfactory operation of this circuit scheme is that the working characteristic shown in Fig. 1 will not be essentially affected by the circuit means here provided, in other words, that the resistance Ra+Ri will be low contrasted with the prevailing apparent resistance of the rectifier. Since the rectifier characteristic here shown is a short-circuit characteristic, the shape oi. curve shown in Fig. 1 will be practically preserved.

Another scheme is shown in Fig. 3. In this instance, the regulated voltage is derived across the rectifiers themselves. Hence, the regulation is governed only by.the voltage division between Bi and the given rectifier apparent resistance. What is especially advantageous in this circuit arrangement is the fact that the rectifier working point, in the presence of loud passages, will be shifted not into the lower knee, but into the region characterized by greater linearity.

The same mode of obtaining the regulated voltage as in Fig. 3 may be employed also in an arrangement according to Fig. 2. The resistance would then be employed for increasing the internal resistance of the generator whereas the impulses would have to be taken oi! directly at the rectifier arrangement.

The use of non-linear resistances is not con fined to the recording of sound in which a compression of the ratio between maximum and minimum volume takes place, but is useful chiefly also for volume control in the reproduction of sound records which, during recording, had been artificially contracted in the sense of an expansion of the volume ratio. The dynamic range in the case of phonograph records amount to 1:50, while the range for sound films is 1:25, unless recourse has been had to special regulation means for increasing the dynamic range (the so-called noiseless method). Inasmuch as the dynamic range in orchestral programs ranges as high as 1110,000, it will be seen that in storing up or recording sound, the limitations imposed upon phonograph records and also films must be borne in mind. In these instances the use oi the socalled "dynamics" or volume-ratio regulator constitutes a substantial improvement upon the quality of reproduction.

Fig. 4 shows a circuit arrangement which is particularly suited for the last-mentioned purposes. The source of alternating voltage T shall here be assumed to consist oi a sound pick-up whose internal resistance shall be denoted by R1. The condenser C blocks the generator part from D. C. In parallel relation to the circuit of the loudspeaker L is branched 01! a small part of the voice energy being available in the output stage. This shunted voltage, which, if desired, may be passed through a combination of nonlinear resistances or additional dynamic regulator comprising the use 01' tubes (forward or rearward regulators) in order that the D. C. voltage obtainable across the condenser C may present the desired amplitude curve, is impressed upon the rectifier assembly G, R, and K. The D. C. voltage arising across the condenser K by way 01' the inserted resistance R is fed to the rectifiers GI and G2 which are connected in a similar way as in the circuit scheme shown in Fig. 2. The variable voltage source B of the circuit scheme Fig 2 is replaced here by the D. C. voltage available across resistance R or condenser K and being a function of the output potential. With increasing value of the output aiternating voltage the rectifiers will receive also an increasing biasing voltage (see Fig. 1). As a result, loud recorded passages will fall inside the region or greater slope, with the consequence that the acoustic record reproduced by the reproducer makes the impression of an expanded volume ratio. The non-linear transmission system N inserted in the circuit oi the branched potential is designed to create an artificial action upon the dependence oi the control D. C. voltage upon the output alternating voltage. This is particularly important in connection with the use of transmission elements having a characteristic that is undesirable i'or regulator purposes, inasmuch as the circuit arrangement such as shown aiIords a chance for supplementariiy influencing the characteristic.

What may be further noted is that in the exemplified embodiments here shown- (Figs. 2 and 3) the chance oi manual regulation oi the biasing voltage by an operator to insure gain control has been kept in mind. In the exemplified embodiment Fig. 4, which is a further development of the scheme Fig. 2, means for automatic regulation of gain or volume have been incorporated.

Instead oi rectifiers as here shown and acting as non-linear resistances, recourse could be had also to other non-linear resistances as before mentioned, such as detectors. iron-cored inductance coils, hot-cathode rectifiers and the like. which must be so designed that by the aid of a biasing voltage a definite working point may be fixed upon its characteristic.

Having thus described my invention. I claim:

1. An audio frequency transmission circuit having a non-linear characteristic and comprising a non-linear resistance element consisting oi two rectifiers conducting in opposite directions, and means for shunting a portion oi the rectified potential from the output circuit to provide a bias on said rectifiers.

2. An audio frequency transmission circuit having a non-linear characteristic and comprising a non-linear resistance element consisting 0! two rectifiers conducting in opposite directions, means for shunting a portion of the rectified potential from the output circuit to provide a bias on said rectifiers, the said means including additional non-linear resistances.

HANS FRIEDRICH. 

